You're joking right? Because of customer support?
Well, customer care is a huge thing for a lot of people (including me).
Back before Amazon opened in Italy, I used a lot of similar services and of course I had problems with any single one of them (like Play.com screwing with my credit card, etc.) including Amazon itself, but the only single time I felt cared was with Amazon. A thing that led me to ditch all the others in favor of Amazon.
I've opened one single ticket with Valve for a technical reason (something I can't remember, related to I think the first Portal when they launched Steam on Mac) and they were late but at least tried to solve my problem with something more than a copypaste answer, so for me for now Steam is fine.
But seeing a lot of these complaints, both here and somewhere else, I'm actually really uncomfortable in case something shitty happens to me, because who the hell knows... :/